Many types of physical systems have been used in experiments investigating mind-matter interactions (MMI). A smaller number of studies have used light as the target of mental influence. Studies involving light are especially useful for studying MMI because light reveals behavior that is distinctively quantum in nature. In particular, it displays wave-like or particle-like behavior depending on how the light is observed. In physics, “observation” is generally regarded as an irreversible amplification of a microscopic event, but there are other forms of observation, including subjective awareness. This observation-dependent effect provides a unique opportunity for studying the meaning of observation and the possible role of consciousness in the physical world.

This talk briefly reviews the relevant literature on MMI experiments using optical systems. Then it focuses on a series of 15 experiments conducted in our laboratory over the past 5 years, all involving optical interferometers. The first study involved a Michelson interferometer, the next ten used a continuous beam Young-style double-slit interferometer, and the last four involved a single-photon double-slit system. The cumulative evidence from these studies suggests that mental attention acts in a manner analogous to an inefficient physical detector; that is, when the detector is operating it causes light to slightly shift from wave- to particle-like behavior. But unlike a physical detector, the mind has the capacity to turn this "detector" on or off through the modulation and direction of attention. I will speculate on this and other interpretations of the experimental results.